Gaseous nitrogen is chemically unreactive due to strong triple bond.
To reduce nitrogen gas to ammonia, a strong enzyme --> reaction is involved, called nitrogen fixation.
Only a few organisms are capable of fixing nitrogen and assembling amino acids.
Higher organisms cannot form NH4+ from atmospheric N2.
Bacteria and blue-green algae (photosynthetic procaryotes) can because they possess nitrogenase.
Enzyme has two subunits:
Strong reductase - has Fe-S cluster that supplies e- to second subunit
Two re-dox centers, one of which is a nitrogenase
Composed of iron and molybdenum that reduces N2 to NH4+
Reaction is ATP-dependent, but unstable in the presence of oxygen.
Enzyme is present in Rhizobium, a symbiotic bacterium in the roots of legumes (i.e. soybeans)
Nodules are pink inside due to the presence of leghemoglobin (legume hemoglobin) that binds to oxygen to keep environment around enzyme low in oxygen (nitrogen fixation requires the absence of oxygen)
Plants and microorganisms can obtain NH3 by reducing nitrate (NO3-) and nitrite (NO2-) --> used to make amino acids, nucleotides, phospholipids.
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